Essential to the understanding of how the tension force in a rope is established through a sheave is the well-known “Rope Friction Formula”: if a tension force F2 is applied to one end of a rope that is spooled about a sheave, the rope will slide off the sheave unless there is a tension force F1 applied to the other end. The relation between the F1 and F2 is given by the “Rope Friction Formula”F2=F1*eμαwhere μ is the coefficient of friction between rope and sheave, and α is the angle in radians taken from F1 to F2 (The radius of the sheave is without influence if the bending stiffness of the rope is low). Given one full turn and μ=0.2, F2=F1*3.5. Given 5 full turns, F2=535*F1. The factor would still be the same with 10 sheaves and half a turn round each in the latter case.
A rope being spooled onto a winch for lifting operations and the like is spooled onto the winch drum at a predetermined tension. Variation of tension of the rope being spooled in layers at the drum may cause serious problems, as the rope in an overlaying layer may “cut into” an underlying layer when a high load is applied to the rope. This will cause problems to the spooling and will also have a detrimental effect on the rope itself. Normally, such an incident makes it necessary to stop the operation to correct the problem.
To avoid this problem, a traction winch is normally used in cases where the tension in the rope to be spooled onto the winch drum is significantly lower than the tension to the rope when the rope is spooled off the winch drum. The traction winch is then dimensioned to have a lifting capacity being equivalent to the maximum lifting capacity of the system. The tension of the rope entering the drum of the winch may then be kept sufficiently constant even if the load of the rope entering the system varies substantially.
Negative features related to existing systems are that the systems are expensive and the systems occupy a large space on board a vessel where space and weight are expensive.
A significant drawback of present tension control systems is the many bending cycles of bending of the rope or cable under load, and in particular when the system is used to compensate for waves (heave compensation). After a heavy payload has been lowered onto the seabed, the cable will be retrieved. When retrieving the cable, heave compensation will not be needed, but the proper tension when spooling the cable onto the winch drum must be maintained. With most present tension control systems, the cable must pass the system regardless whether there is a need for tension control or not.
Also, a major drawback with existing tension control systems is that they are not easily integrated with existing winches.
Cables, and in particular those utilized in the offshore industry, add an extra challenge with respect to spooling, as they may have a main body interspersed with segments of diverse cross section and/or bending stiffness due to integrated devices like hydrophones or magnetic sensors (hereinafter “segmented cables”). These segments may have a cross section that does not fit the groves in sheaves and drums laid out for the main bulk of the cable, and such segments may easily become damaged if they are bent beyond a limit. In operation, segmented cables are spooled onto a drum of large diameter (e.g. 4 meters or more) at low tension to avoid excessive bending.
WO 2011/139160 A1 discloses a tension control device for an anchor line rope of large diameter having two movable link arms connecting to a fixed link arm, the movable arms being operated by hydraulic cylinders. Each link arm is carrying a sheave operated by a hydraulic motor. However, WO 2011/139160 A1 does not allow for attaching to a cable under tension and does not allow for segmented cables without significantly bending the cable.
An object of the present invention is to provide a solution where the above-mentioned problems are solved. Other objects of the present invention will be obvious after studying the present description.
In the present description the term “rope” has been used for an elongated body. The elongated body mentioned by the term “rope” may be a rope, a wire or a cable. Presently, the most preferred elongated body is a rope, especially a fibre rope made of synthetic fibres.